• The anthrax bacterium Bacillus anthracis determines from high external CO2 concentration (about 5%) that it has arrived in the lungs or bloodstream of its mammalian host. It then prepares for dispersal by producing toxic proteins to kill the host and synthesizing a protective capsule around itself. [1]

• Pathogenic fungi also employ CO2 concentration as a means for detecting that they have arrived in the appropriate host tissue. Candida albicans and Cryptococcus neoformans, which cause life-threatening systemic infections in immunocompromised patients, synthesize protective capsules and scavenging filaments in response to CO2 levels of around 0.5%. [2]

• Mushrooms elevate their fruiting bodies (caps) to an optimal height for spore dispersal based on measurements of atmospheric CO2 concentration. [3] Root and microbial respiration drives CO2 concentration near the soil surface to above 0.1%. Exposure of mushrooms to such CO2 levels promotes stalk elongation instead of cap expansion. Only when the fungus reaches heights above the soil, where CO2 concentration drops below 0.05%, does cap expansion take precedence. [4]

• Social insects such as ants, bees, and termites detect the location and activity of hives via gradients of CO2 concentration. Bees also determine whether ventilation in a hive is adequate from CO2 concentration: If concentration in a beehive rises above 0.5%, young worker bees orient in a single direction and beat their wings to ventilate (fan) the hive.

• Insect herbivores may monitor CO2 concentration to identify the most delectable plant tissues: The tissues that are most rapidly depleting atmospheric CO2 are synthesizing the most carbohydrates. [5] For example, a moth aptly named Cactoblastis cactorum, which feeds on prickly pear cactus (Opuntia sp.), resolves differences as small as 0.00005% (0.5 ppm) CO2 against the normal atmospheric background of 0.0386% (386 ppm) to locate in the dark the cactus pads that are most active in CAM carbon fixation (see Figure 5.12).

• Haematophagous insects such as mosquitoes, ticks, and tsetse flies find their next blood meal by following the trail of CO2 to their prey (Figure 5.32). These bloodsuckers activate flight or searching behavior at a relatively low CO2 concentration gradient, 0.001 to 0.030% (10 ppm to 300 ppm), but begin to fly in the direction of their prey only at a higher concentration gradient of about 0.1% (1000 ppm). [6] Many insect repellants, including DEET (meta-N,N-diethyl toluamide), work by blocking insect CO2 receptors. All of these organisms seem to rely on similar biochemical mechanisms for CO2 sensing. [7] Will rising CO2 concentration in the atmosphere interfere with any of these processes? The anticipated changes of 0.05% to 0.10% CO2 could present difficulties for insect herbivores and haematophagous insects that will be forced to detect a CO2 signal against a higher background level. Other climate changes, however, may expand the ranges of these insects.